To clarify the correlation between erotic and nocturnal erections, and to evaluate the diagnostic efficacy of the audiovisual stimulation penogram as an initial screening test for impotent patients, a comparative study of 137 impotent patients was designed. The audiovisual stimulation penogram shows dynamic penile blood flow change during audiovisual stimulation using a radioisotope (99mtechnetium) and is classified as type 1--similar findings to normal volunteers, type 2A--impossible erection due to insufficient blood flow, type 2B--unstable erection due to severe fluctuation in blood flow and type 2C--delayed erection. The compatibility ratios of the audiovisual stimulation penogram types 1, 2A and 2C with some tests for nocturnal erection were relatively high (63 to 80%) and when compared to the final diagnosis all but 1 of the compatible cases with nocturnal erection studies were able to be diagnosed correctly. However, the compatibility ratio of the audiovisual stimulation penogram type 2B was markedly low (35%) and this group had complicated etiologies, including many more psychogenic than organic causes. Although the mechanisms of the erotic and nocturnal erections have not been fully elucidated to date, our results suggest that the audiovisual stimulation penogram seems to be an appropriate initial screening step if we refine the criteria for type 2B, and a comprehensive interpretation of the tests for erotic and nocturnal erections will result in a more accurate diagnosis.